The present invention relates to a structure for measuring a large weight with high precision, and more particularly to a device for measuring a large vertical force such as a great weight using a load pick-up having a force transducer and a weighing cell.
In accordance with the invention, a force transducer is used wherein a force such as delivered by a large weight is resolved into components and is mechanically or hydraulically transmitted and converted into analog electrical voltage units or digital pulses and is thereby converted, edited and made detectable in a suitable manner for a measurement of the force or weight.
Structures heretofore have utilized a force resolution wedge as a force transducer with a fluid work agent supplied under pressure and an interactive connection to a pair of support elements having gliding surfaces disposed in series with a final control element as a load chain whereby the load chain is equipped with a force measuring means as shown in German OS No. 27 58 430.
The advantage of such arrangement equipped with a force resolution wedge, a final control element and a force measuring means resides in that the arrangement achieves the following:
is uncomplicated in structure and function;
is employable up to extremely high loads;
is practically friction-free in operation;
is suitable for executing a regulating distance under load;
is optimum in damping or in attenuation behavior.
In view of the foregoing advantages, the general utilization of load resolving wedges have not been successful. For example, in weighing technology this is due to certain difficulties such as the requirement of all-around guidance of the wedge and due to the necessity of a dust-tight encapsulation of the oil conducting parts. The lack of complete success has also been due to the requirement of making an extensive offering or availability of resolving wedges having different leads and dimensions available for different load ranges and also due to the difficulty of designing a hydrostatic wedge arrangement to form a compact self-contained machine element.
It is accordingly an object of the present invention to provide a structure which retains the advantages of a force resolving wedge and yet obtains an improved weighing device having a force transducer and a weighing cell and force measuring member which are formed into a compact load cell which can be manufactured by economical means and is capable of covering a wide load range using only a few types of standardizable and normal components or individual elements. It is also an object of the present arrangement to provide a structure which produces no external forces which must be compensated by support elements and which has a structure capable of utilizing a dust-tight encapsulation which is not complicated and can be employed for use in a great many branches of industry, particularly which is capable of measuring great forces with the highest precision.
A feature of the invention is to provide a weighing appliance of the type above referred to where the force transducer is designed as a hydrostatic spindle and nut unit having helical work surfaces which convert an axial or weight force into a torque effective at right angles relative to the direction of force or weight and employing a weighing cell connected to a force measuring member through a torque arm.
The structure embodying the principles of the invention fully utilizes or exploits the advantages of a force resolving wedge and on the other hand, overcomes difficulties existing in devices heretofore available. Such difficulties are avoided as in the necessity of compensating for external forces by a support element. Further, an extremely wide load range is covered without creating problems, and the device makes it capable of having a structural series of standard and normal modules. With this arrangement, at least one power of 10 is covered in the load range with the length of a torque arm which can be adjusted as desired. Further, the spindle and nut arrangement is suitable dust-tight encapsulation with uncomplicated structure. Further, because of its compact and self-contained structure, the unit is employable as an element in a great many branches of industry where large weights must be measured or where large forces must be measured with high precision.
It is a feature of the invention that a hydrostatic spindle and nut arrangement is utilized which is connected to a supply system for a hydrostatic parting liquid supplied under pressure. The unit employs helically inclined work surfaces which have a lead between 1:10,000 and 1:10, but preferably in the range of between 1:1,000 and 1:100. With the utilization of helical work surfaces with an extremely slight lead and hydrostatic liquid between the surfaces an essentially friction-free weighing device is accomplished which develops a high mechanical resolution when resolving the weight force and the apparatus is successful in weighing even the heaviest loads with high precision. Because work surfaces with such slight leads require processing precisions having surface qualities in the micron range, the problem of economical fabrication feasibility is increased. This is resolved in one form by manufacturing the work surfaces by coating metal surfaces of the spindle and nut unit with a pressure injection method. With this method, a molding is manufactured for a work surface with the identically equivalent cooperating work surface which enables economical shaping of the work surfaces of a synthetic material, and with the utilization of the pressure injection method, the surfaces can be produced with high precision.
In accordance with the present invention, the apparatus includes generally the following parts:
a stationary member including a lower part or base body with an upper helically shaped work surface and a concentric central bore for the acceptance of a centering shaft;
a nut freely resting against the base body having on its underside a helically extending work surface cooperating with the work surface of the base body and having an upper side having a helically shaped work surface and a concentric bore in the center receiving a centering shaft;
a centering shaft disposed in the bores with the base body and nut and guided in friction-free contact therewith by hydrostatic parting surfaces;
an upper part to the stationary member which is designed as a load pick-up and which on its underside has helically extending work surfaces coacting with the work surfaces of the nut with hydrostatic bearing means therebetween;
a torque arm radially disposed at the nut;
a stationary force measuring member connected to the torque arm;
a system of channels for delivering and removal of hydrostatic parting liquid to the centering shaft and to the helical work surfaces.
An arrangement of this embodiment resides in the identical configuration of the work surfaces of the base body and nut which improves the ability to manufacture the unit and reduces manufacturing costs. Further, due to the disposition of the centering shaft which is equipped with hydrostatic parting surfaces, an exact and reliable, and therefore contact-free and friction-free guidance of the spindle body and nut body is achieved. Further, the upper stationary member, as a result of its friction-free horizontal displaceability by means of hydrostatic parting surfaces, transmits an exact load pick-up even with a slight eccentric offset of a load.
As a result of the space-saving compact structure of the spindle and nut unit which externally preferably is in the shape of a cylindrical block, provides for more ease of sealing with an elastic gasket to prevent the escape of hydraulic fluid.
Further advantages are obtained in the error-free force resolution of the supported load even with a force introduction which may not be precisely centered. This is accomplished in that the work surfaces of the base and nut are subdivided into at least two, and preferably three, helically designed subsurfaces which are grouped around a common center similar to a three-leaf clover.
In another form of the invention, the weighing apparatus utilizes at least two hydrostatic spindle and nut units having torque arms which are interconnected to form a load chain which is connected to a force measuring member.
Other objects, advantages and features, as well as equivalent structures which are intended to be covered herein, will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments in the specification, claims and drawings in which: